As one of excellent user interface means, a touch panel which detects a touch of a finger or the like and specifies the position coordinates thereof has received attention.
and various types of touch panels such as a resistance film type and a capacitive touchscreen system are commercialized.
The capacitive touchscreen system includes a projected capacitive touchscreen system capable of detecting a touch even if the front surface side of the touch screen in which a touch sensor is incorporated is covered with a protection plate such as a glass plate having a thickness of about several mm. This system has the advantages that excellent fastness can be achieved since the protection plate can be arranged on the front surface, that detection of a touch is possible even when a user wears gloves, that long-life is ensured since this system has no movable part, and the like.
The touch screen of the touch panel disclosed in Patent Document 1, for example, comprises a conductive element of the first series formed on a thin dielectric film and a conductive element of the second series formed with an insulating film interposed therebetween as detecting conductors for detecting an electrostatic capacitance, and there is no electrical contact between these conductive elements but a plurality of intersection points are formed therebetween. The materials best suitable for the conductive elements are metal materials such as silver. Further, the material depends on the required visibility of display, and if the visibility is intended to be low, indium oxide is used. Instead of the conductive element, a fine wire of 10 μm to 20 μm can be used. The conductive elements for detecting the electrostatic capacitance are connected to a capacity control oscillator through an output line and a multiplexer. The output therefrom is counted by a divider and becomes capacitance detection data. With the relative values of the detection capacitances of one or more conductive elements, the touch position between the conductive elements can be interpolated.
Patent Document 1: National Publication of Translation No. 1997-511086 (line 19 of page 7 to line 4 of page 8, line 23 of page 8 to line 6 of page 9, lines 4 to 12 of page 13, line 23 of page 13 to line 10 of page 14, FIGS. 1, 2, 6, and 8)
As such a capacity control oscillator as discussed above, a relaxation oscillator can be used. The oscillation cycle of the relaxation oscillator generally depends on the charge/discharge time constants of a resistance element and a capacitive element. By utilizing the electrostatic capacitance (hereinafter, referred to as a “touch capacitance”) formed between a detection wire and a finger of a user or the like (hereinafter, referred to as an “indicator”) to form part of the capacitive element, if the indicator makes a touch, there arises variation in the oscillation cycle of the relaxation oscillator according to the touch capacitance formed between the detection wire and the indicator. By detecting the variation of the oscillation cycle, the touch capacitance is detected and, with the touch capacitance detected at, for example, adjacent detection wires, it is possible to calculate the touch position between the adjacent wires as touch coordinates by interpolation.
In order to ensure interpolation of the touch coordinates with high accuracy, it is important to increase the capacitance detection sensitivity of the detection wires around the touch position.
If the capacitance detection sensitivity is set uniformly for all the detection wires, however, when the number of detection wires increases, the detection time accordingly increases and the detection time cannot satisfy a desired detection time (generally, 100 ms or shorter). This causes an unpleasantness in operation. If the desired detection time is intended to be satisfied, the capacitance detection sensitivity for one detection wire required for interpolation cannot be ensured and this arises a problem that the touch coordinates cannot be accurately calculated.